Most of us in the Puget Sound area are aware that the iconic Southern Resident Orcas and the food chain that supports them are exposed to toxic contaminants, habitat loss, hydropower dams, vessel strikes, noise pollution, ocean acidification, climate change, and overharvesting of Chinook salmon – their primary source of food. But there may be another threat lurking in our waters that is relatively unnoticed: invasive zooplankton.
Invasive species have the ability to significantly alter ecosystems and contribute to declines of native species through competition and predation. As of 2004, there were 50,000 documented species of invasive plants and animals in the U.S. causing roughly $120 billion dollars in damaged ecosystem services and contributing to the endangerment of native species. Concern over the food web impacts of invasive species in West Coast estuaries has been rising. For example, in their 2009 biological opinion regarding the Central Valley Project and State Water Project in Central Valley, California, the National Marine Fisheries Service (NMFS) identified the invasive Asian clam, Corbicula fluminea, as a threat to salmon abundance in California. As a very efficient filter feeder, C. fluminea can reduce the local abundance of phytoplankton, causing potentially negative effects on zooplankton and their salmonid predators.
Estuaries along the west coast of North America are among the most invaded in North America. Many of the invasive species are zooplankton. Zooplankton are small, mostly free-swimming animals such as those pictured in Figures 1 and 2. Along with the phytoplankton on which they feed, zooplankton form the base of marine and aquatic food webs. In North American lakes and in other parts of the world, zooplankton invasions have corresponded with declining native zooplankton and fish populations.
At one point, the San Francisco Estuary was thought to be home to the largest number of invasive species in the world. By 2011, the San Francisco Bay Estuary alone was home to nine invasive zooplankton species. Invasions by species from Asia were associated with declines in native and historically dominant copepods. One of those invasive species, Oithona davisae, was found to be among the most seasonally abundant zooplankton species in the San Francisco Bay.
It is likely that these invasive zooplankton traveled across the Pacific in ballasted ships. Ballasted ships need to take on water when they are not carrying cargo to stabilize them during transit, and ballast water is one of the primary vectors of marine species invasions.
In 2012 a large population of the copepod Oithona davisae was discovered in Samish Bay in Northern Puget Sound, likely transported in ballasted ships travelling domestically from California. There, Oithona davisae seemed to replace native species. Oithona davisae are smaller than native copepods and may be less susceptible to capture by juvenile fish. According to zooplankton researcher Jeff Cordell, their presence in high numbers could influence food webs by reducing the prey available to juvenile fish, including salmon. Whether or not this invasive species will have significant cascading effects on food webs in the Puget Sound remains to be seen. However, because invasive zooplankton have corresponded with declines in important fish species elsewhere and contributed to species’ endangerment, we should be concerned about the management of invasive zooplankton and how their presence could impact the Southern Resident killer whales who are already vulnerable to so many anthropogenic threats.
Orcas received a lot of attention in Governor Inslee’s 2019-2021 Biennium Budget. The Governor’s budget allocates millions towards various methods for decreasing disturbance from vessel traffic, reducing toxic contaminants, and increasing Chinook salmon populations, their primary source of food. While much of the budget is devoted to those three issues, the budget does allocate $1.4 million for monitoring zooplankton as part of one of its ‘Science and Support’ activities to fill knowledge gaps to help support recovery of Southern Resident Orcas. At the same time, evidence suggests that eradication of invasive zooplankton is largely unachievable. This emphasizes the importance of preventing future invasions.
The International Maritime Organization’s 2004 International Convention for the Control and Management of Ships’ Ballast Water and Sediments went into effect in 2017 and the United States Coast Guard passed similar rules in 2012. The shipping industry has been developing and implementing different technologies and methods to treat ballast water in order to comply with new standards on the amount of viable or living zooplankton in ballast water discharge. These new methods may curb the rate of invasions. More research could help us learn which methods are most effective.